The release of drugs from an implanted medical device has been shown to be beneficial for the function of devices and the treatment of various medical conditions. For example, delivery of a drug from the device surface can prevent cellular responses initiated by the presence of the implantable device. Also, drug released from the device can prevent conditions that would otherwise shorten the functional life of the device following implantation. Drug released from the device may also be directed at treating a diseased area of the body.
Some implantable devices simply have a drug applied to the device surface. Such preparations are generally undesirable because the drug can be easily removed from the surface during insertion. In addition, release of the drug is generally difficult to control following implantation.
Implantable medical devices having thin polymeric coatings containing therapeutic compounds have been described in the art and provide improvements for protecting and controlling the release of drug from the device surface. Some of these coatings are capable of releasing drugs to provide a local therapeutic effect in the vicinity of the implanted device. Such devices have been shown to be particularly valuable for the treatment of diseases of the cardiovascular system.
Drug-eluting stents can provide localized release of a therapeutic substance at the site of administration. Local administration of therapeutic agents via polymeric coatings on stents has shown favorable results in reducing restenosis. Several classes of polymer chemistries have been explored for use in drug-releasing coatings for stent as found in current art, some of which have been approved and are currently being used in medical procedures. Many of these chemistries are useful for delivering hydrophobic drugs.
Drug-releasing coatings are typically prepared using a coating composition having a drug and a polymer dissolved in solvent. The composition is then applied to a substrate surface and the applied material is dried to remove the solvent, which results in a coating of polymeric material with entrapped drug that can be eluted from the coating following implantation.
For example, coating compositions based on poly(alkyl(meth)acrylate) and poly(ethylene-co-vinyl acetate) mixtures suitable for preparing coatings for hydrophobic drugs (such as rapamycin) release are described in U.S. Pat. No. 6,214,901. Release of hydrophobic drugs in a controlled manner can be achieved using this type of polymeric coating system. For example, this system provides sustained and controlled release of the hydrophobic drug, wherein less than 50% of the total quantity of the hydrophilic drug released is released in the first 24 hours.
Another hydrophobic polymer system stated to be useful for drug delivery is described in U.S. Pat. No. 6,669,980, which teaches preparation of medical devices having coatings that include poly(styrene-isobutylene-styrene).
Yet other hydrophobic polymer systems useful for drug delivery are described in U.S. Patent Publication Nos. 2005/0220843 and 2005/0244459.
For certain medical applications, these polymer systems are not ideal. For example, some applications involve the transient insertion of a medical device to a target tissue in the body. For the polymer systems described above, the rate of release of drug from such a polymer system may not be sufficient to provide a therapeutic amount of drug to the target tissue.
In addition, many of the drug delivery coating are made for devices with “static surfaces”, that is, surfaces that do not increase in area. Typically, polymer systems that form durable coatings are suitable for these static surfaces. However, on surfaces that are non-static (e.g., elastic surfaces) such durable coatings may not always be appropriate.